1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same. For example, they are suitable for a digital still camera and a digital video camera.
2. Description of the Related Art
Recently, a single lens reflex camera is required for having a video recording function and performing autofocus during recording a video. A contrast system evaluating focusing state of an image pickup optical system by detecting high frequency component in an image pickup signal is widely used as an autofocus system during recording a video.
It is important to properly select a zoom type and the number of a focus lens unit in order to acquire a zoom lens which has small aberration variations during focusing and high optical performance over general object distances and can perform high speed focusing.
Japanese Patent Laid-Open No. 11-202203 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power and a fifth lens unit having a negative refractive power, and moves the first lens unit during focusing. However, when the first lens unit which has the longest effective diameter and is heaviest in all lens units is moved for focusing, focusing speed becomes slower and driving noise of the focus lens unit becomes louder.
U.S. Pat. No. 8,068,281 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power, a fifth lens unit having a negative refractive power and a sixth lens unit having a positive refractive power, and moves the fifth lens unit during focusing. However, since one lens unit only moves during focusing, variation of optical performance by focusing becomes larger, and especially the optical performance tends to decrease when focusing to a close distance object. Additionally, at a telephoto side, a movement amount of the focus lens becomes larger, and as a result focusing speed tends to become slower.
Japanese Patent Laid-Open No. 2000-047107 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, a fifth lens unit having a positive refractive power and a sixth lens unit having a negative refractive power, and moves the fourth lens unit and the sixth lens unit during focusing. However, at a telephoto side, optical performance varies according to object distance and a movement amount of a focus lens unit tends to become larger.